This invention relates generally to methods of and apparatus for removing odors from airstreams. More particularly, this invention relates to methods of and apparatus for neutralizing odors in a relatively low temperature airstream by removing malodorous constituents from the airstream with finely divided liquid droplets of reagents with which the constituents react.
Offensive odors are characteristic of many chemical process industries and are often difficult and costly to eliminate or control. Of particular interest with respect to the instant invention is the technology of controlling odors emitted by waste water sludge composting facilities, wherein the sludge is biologically degraded into a stable end product. The end product is a sanitary, odor-free humus-like material which is used as a soil conditioner for yards, gardens, and athletic fields. By composting waste water sludge, a material which until recently was considered to be a waste, has become a valuable soil conditioning product which is generally less costly than the top soil or peat moss it replaces yet produces better results. Wide utilization of this resource has been curtailed because composting facilities typically produce undesirable odors. The problem is compounded because composting sites are most conveniently near waste water treatment plants, which themselves are located in or near urban or suburban areas. It is, therefore, necessary to minimize the odor emitted by composting facilities. This invention is directed to that end.
While the technology disclosed in the instant application is of particular significance with respect to waste water sludge compost facilities, it also is applicable to industries such as, but not restricted to, petroleum refineries, rendering plants, pet food producers, flavor and fragrance producers, and fermentation processes. While there are presently no practical federal regulations for odor control, local laws and local political pressure is often sufficient to discourage introduction of such process industries; and, if industries are already established, local laws are sufficiently stringent to enable authorities to act on complaints and level stiff fines and, in some cases, even close plants.
In view of these considerations, a number of approaches have been tried to control odors. The simplest is merely to dilute odorous gas with large quantities of air. Another approach has been to mask offensive odors with more pleasant ones, but this is not particularly successful in that, to some people, the new odor is as unpleasant as the original odor; and, downstream of the plant, the mixture is often not sustained. In any event, any odor emanating from a plant, whether pleasant or unpleasant, is suspect. Incineration has been attempted, but this is a very costly approach in that an entire odorous gas stream must be subjected to combustion temperatures In a situation where the process has no combustion stage or the airstream is saturated with moisture, such as with process air from a composting system, combustion for odor control is not at all practical.
Another common approach to odor control is by chemical neutralization of odorous compounds. The odorous gas is conventionally washed with an aqueous solution of a reactive chemical in spray towers, packed beds, and the like. Chemicals commonly used for this purpose include permanganates, dichromates, acids, hypochlorite solutions, hydrogen peroxide, and other common oxidizing agents. Exemplary processes are described in British Patent No. 1,152,705 and U.S. Pat. No. 3,923,955. Another approach for chemical neutralization of odors is described in U.S. Pat. No. 4,125,589, in which odoriferous constituents are removed from essentially saturated gas streams utilizing relatively small volumes of liquid in very finely divided droplet form with relatively long gas-liquid contact times. This process was improved upon by the process described in U.S. Pat. No. 4,238,461, wherein the droplet size was further controlled. Further exemplary of the prior art are U.S. Pat. Nos. 4,225,566 and 4,416,861.
U.S. Pat. No. 4,225,566 utilizes two or more liquid reagents, with one being dispensed above another in a reaction vessel through which waste gas is passed, while U.S. Pat. No. 4,416,861 utilizes a two-stage odor control system, wherein exhaust air from fat-rendering plants is fogged with sulfuric acid.
Utilization of the foregoing technology has resulted in removal of a substantial portion of the odor in process air from a composting facility with which the instant inventors have worked. However, the remaining odor is still sufficiently pervasive to precipitate a substantial number of complaints from citizens. While this remaining odor does not present a nuisance which is sufficient to shut down the plant or result in fines, it can affect the plant's reputation and can hinder the erection of more composting facilities. In that it is to everyone's benefit to erect and utilize composting facilities for management of sewage sludge, substantially controlling all odor is of utmost importance if this concept is to gain wide acceptance.
In controlling the remaining odor-causing constituents, the inventors fogged the process air with a surfactant solution. This is a unique concept, not taught in the prior art for applications such as a composting facility or other large processing operation. However, the prior art does disclose utilizing detergent to wash odors and grease from cooking stove hoods, as is exemplified in U.S. Pat. Nos. 3,841,062; 4,351,652; and 4,753,218. There is no suggestion in these patents that fogging with surfactants in combination with other scrubbing reagents would result in improved removal of the remaining odor-causing constituents from a stream of relatively low temperature air which has odor-causing constituents generated by a sludge composting system or similar facility.
In view of these and other considerations, there was a need for methods of and apparatus for removing odor-causing constituents from airstreams, which constituents remain as residuals when prior processes are utilized.